In the first study of its kind since the 1920s, rats in New York City were found to carry a flea species capable of transmitting plague pathogens.

In research appearing March 2 in the Journal of Medical Entomology, researchers led by Dr. Matthew Frye, an urban entomologist with Cornell University’s New York State Integrated Pest Management (IPM) Program, reported collecting more than 6,500 specimens of five well-known species of fleas, lice and mites from 133 rats. Among them: 500-plus Oriental rat fleas, notorious for their role in transmitting the bubonic plague, also known as the Black Death.

The team looked most closely at the rat flea because of its potential as a vector for human diseases. Besides their role in transmitting plague bacteria (called Yersinia pestis,), the rat flea is also known to transmit the pathogenic bacteria Rickettsia and several species of Bartonella, which Dr. Frye says “can cause a wide range of clinical syndromes, some severe.”

Dr. Frye’s colleagues at Columbia University’s Center for Infection and Immunity used molecular screening methods to search for these pathogens in the collected samples. The researchers did not detect Rickettsia or plague, but they find Bartonella.

The most important finding, however, is that the vector — the rat fleas — is present, which indicates the potential for disease transmission, says Dr. Frye. “If these rats carry fleas that could transmit the plague to people, then the pathogen itself is the only piece missing from the transmission cycle,” he says.

Plague can be found in some regions of the US, such as the American Southwest, where it infects an average of 10 people each year. The pathogen is transmitted in these areas by the fleas of ground squirrels and prairie dogs. In other parts of the world, the incidence of plague is higher.

However, New Yorkers shouldn’t be too worried about an imminent outbreak of Black Death in the Big Apple. Commenting on the study, the city’s Health Department emphasized that plague-carrying rats have never been found in New York. “Plague requires extreme circumstances besides fleas to pose a threat to human health, and those circumstances do not exist here,” a Health Department spokesperson told the New York Daily News.

But that doesn’t mean the city’s rats are harmless — previous research conducted by Dr. Frye on the same 133 rats found that the rodents carried a “disturbing number” of viral and bacterial diseases. These included previously undocumented pathogens that could infect humans.

Dr. Frye recommends that New Yorkers remove food and water and prevent access to shelter in an effort to hold back rodent infestations. Also, when evicting rats from homes and workplaces, Dr. Frye says it is crucial that careful sanitation is followed to remove the fleas, lice and mites left behind by the rodents. “It’s not that these parasites can infest our bodies,” he says, “but they can feed on us while seeking other rats to infest.”